The opioid neuropeptide system plays an important part in regulating mood disorders. [Machado-Viera R. et. al.; Depression and Anxiety, 28(4) 2011, 267-281]. Opioid peptides and their receptors are potential candidates for the development of novel antidepressant treatment. The actions of endogenous opioids and opiates are mediated by three receptor types (μ, δ and κ), which are coupled to different intracellular effector systems. [Berrocoso E. et. al., Current Pharmaceutical Design, 15(14) 2009, 1612-22]. As such, agents that can modulate the actions of one or more of the opioid receptor types with selectivity and sensitivity are important to treat the various diseases and disorders regulated by the opioid system.
The μ-opioid system has a profound effect on emotional state and is modulated in the context of major depressive disorders (MDD) and changes in emotional state. The μ-opioid receptors are present and densely distributed in brain regions implicated in the response to stressors and the regulation and integration of emotionally significant stimuli. These include cortical regions, including the rostral anterior cingulate, prefrontal cortex [Eisenberger, Science 302, 2003, 290-2; Kennedy Arch Gen Psychiatry 63(11), 2006, 1199-208; Zubieta, Science, 293 2001, 311-5; Zubieta, Arch Gen Psychiatry, 60(11), 2003, 1145-53]. Subcortically, the μ-opioid system is known to have a prominent regulatory role in the striatopallidal pathway (nucleus accumbens, ventral pallidum) and associated circuits (e.g., amygdala, thalamus, insular cortex) involved in the evaluation and response to salient stimuli, both rewarding and nonrewarding [Anderson A K, and Sobel N. Neuron 39(4) 2003, 581-3; Horvitz J C., Behav Neurosci. 114(5), 2000, 934-9; Koob and Le Alcoholism Clinical & Experimental Research, 2001 25(5 Suppl.) 2001, 144S-151S; Napier and Mitrovic, Ann N Y Acad. Sci., 1999, 176-201; Price 2000; Quirarte, Brain Res., 808(2), 1998, 134-40.; Steiner and Gerfen, Exp Brain Res., 60-76, 1998; Zubieta, Science, 293 2001, 311-5]. Activation of μ-opioid receptors increases dopamine which may contribute to anti-depressant effects including enhancement of hedonic tone and sense of contentment, but will lead also to abuse when the increase in dopamine is higher than required to treat symptoms of depression.
Positron emission tomography (PET) studies in humans have shown functional effects of the μ-opioid system in the regulation of mood. In vivo μ-opioid receptor availability in the sub-amygdalar temporal cortex has been found to inversely correlate with the metabolic responses of this region to the presentation of a negative emotional challenge [Liberzon, Proc Natl Acad. Sci. 99(10): 2002, 7084-9]. In a subsequent PET study emotional challenges were shown to elicit further differences in brain μ activity between normal human subjects, patients with SSRI responsive MDD, and patients with treatment resistant depression [Kennedy, Curr. Psychiatry Rep. 8(6), 2006, 437-44].
It has been hypothesized that blockade of κ-receptor activation will have a beneficial therapeutic effect in the treatment of depression. The hypothesis is based on human and animal evidence generated primarily during the past two decades. The following discussion is adapted from a recent review by Knoll and Carlezon, Jr. [Brain Res. 2010, 56-73, 2010]. Whereas μ-opioid receptor activation results in elevation of mood in humans, activation of the κ-opioid receptor is associated with adverse effects on mood, including dysphoria and anhedonia [Pfeiffer, Horm Metab Res., 18(12): 1986, 842-8].
Anatomically, the κ-opioid receptor and dynorphin, the primary endogenous κ ligand, are expressed throughout limbic brain areas implicated in the pathophysiology of depression. In addition to dysphoria and anhedonia, some aspects of the aversive effects of κ activation appear to involve increased anxiety. κ-opioid receptors and dynorphin are expressed throughout brain areas involved in fear and anxiety, including the amygdala and extended amygdala (Alheid 2003; Fallon and Leslie 1986; Mansour, 1995b]. The effect of κ blockade in humans has yet to be tested in humans; a pharmaceutically acceptable probe has eluded medicinal chemistry efforts.
Treatment resistant depression (TRD), is a widespread disease where patients with MDD do not achieve an adequate response to monoamine reuptake inhibitor anti-depressant therapy. Despite the emergence of multiple new therapeutic agents in recent decades, TRD remains a major clinical and public health problem that results in significant adverse consequences to patients, families, and society as a whole [Gibson, J., Manag. Care, 16:370-377, 2010; Sackeim, J Clin Psychiatry, 62 Suppl 16:10-17, 2001]. Prior to the advent of monoamine oxidase inhibitors (MAOIs) and tricyclic antidepressants (TCAs), opioids were the primary therapeutic modality for depression. Modern characterization of the endogenous opioid system has elaborated the role of opioidergic peptides in the regulation of both stress response behaviors and hedonic tone. Buprenorphine, a partial μ-opioid agonist, has been reported to be useful in treating depression in patients where other available therapies have failed. [Callaway, Soc. Biol. Psychiatry, 39, 1996, 989-990; Emrich et. al., Neuropharmacology, 22, 1983, 385-388; Bodkin et. al., J. Clin. Psychopharmacology, 15, 49-57, 1995].
While opioid agonists have anti-depressant effects they are generally not used to treat depression. Long-term use of a full μ-opioid agonist may result in the development of opioid-dependency in patients. In addition there are other undesirable side effects including additive potential, sedation, respiratory depression, nausea and constipation that will accompany acute and chronic opioid use. Buprenorphine is an μ-opioid partial agonist which produces typical μ-opioid agonist effects and side effects such as additive potential and respiratory depression while producing maximal effects that are less than those of full agonists like heroin and methadone. Buprenorphine produces sufficient μ-agonist effect to enable opioid-addicted individuals to discontinue the misuse of opioids without experiencing withdrawal symptoms.
While there are many well-known opioid receptor binding compounds, there is little evidence to guide the management of depression that has not responded to a course of antidepressants. Treatment-refractory depression is an important public health problem and large pragmatic trials are needed to inform clinical practice. [Stimpson et al., The British Journal of Psychiatry, (2002) 181: 284-294]. There still remains a need to develop effective treatments of mood disorders, in particular major depressive disorders.